Error prevention in a recording and reproducing device with at least one rotating head

ABSTRACT

A recorder/reproducer having two capstans which engage a magnetic tape passing from a delivering coil holder to a receiving coil holder of a cartridge is disclosed. At the time of loading the tape, the capstans are shifted from a rest position along a path toward the scanner which has an upper, rotatable drum. In this working position, the capstans are in close proximity to the scanner which eliminates the great lengths of the tape which generate undesired longitudinal vibrations in the tape.

TECHNICAL FIELD

The present invention relates to a recording device such as a magneticrecording device with rotating head or heads with helical scanning.

PRIOR ART

The recording of data, whether analog or numerical, is almost alwaysexecuted within a layer of very small thickness deposited on a supportin the form of a flexible film. This is true for, e.g., writing onpaper, photorecording and, above all, of the magnetic recording.

The analog magnetic recording of sound necessitates a relatively smallspeed between head and tape. On the other hand, since the signal/noiseratio of the recording is proportional to the utilized surface of thetape, the recording track has to be sufficiently wide. This is thereason why in the usual process, the tape moves in front of a stationaryhead.

For analog recording of a video signal or any numerical recording, thesignal/noise ratio is less critical which permits the width of therecorded track to be greatly reduced. On the other hand, since thenumber of magnetic transitions per second is very high, one has toforesee a great relative speed between head and tape. For example, theprofessional analog recording of sound is generally executed at 19.05 or38.1 cm/sec on a width of 2 to 6 mm, while the professional analogrecording of a video signal (format C) is executed at a relative speedbetween head and tape of 25 m/sec but on a width of only 0.16 mm. Such anarrow tape moving at such a great speed is not practical. Therefore,since about 1950, the universally adopted solution for the recording ofvideo signals in practice, makes use of mobile, rotatable heads whichscan the surface of the magnetic tape.

Originally, this scanning was executed transversally, i.e., where theaxis of rotation of the rotating head or scanner was parallel to thedirection of the movement of the tape (format Quadruplex). Today, almostall new recording devices make use of helical scanning, i.e., where thetape moves helically about a cylinder, a portion of which is generallystationary and another portion of which is rotatable. This rotatablepart comprises one or more magnetic heads which, due to the helicalwinding of the tape, scan the surface of the latter diagonally. Thelongitudinal component of the motion of the tape has the effect that therecorded tracks are not superimposed but that they are arranged likehatchings in a drawing.

In the devices actually known the tape is driven by the receiving coilof the tape and by a capstan in the form of a cylinder generallyrotating at constant speed. Between these elements and the scanner, onefinds various elements, like magnetic heads, provided for the auxiliarylongitudinal tracks, a counting roll measuring the longitudinal path ofthe tape and various tape guides, among them the input and output guidesof the scanner. These latter guides are generally stationary but incertain cases they can be rotatable (e.g., Ampex Nagra VPR 5).

The preceding shows that in all known cases there exists between thescanner and the capstan a relatively great length of tape. The rotationof the scanner, combined with the frictional effects in the guides,causes longitudinal vibrations in the tape which are only stopped by thecapstan. These vibrations deteriorate the quality of the signal of theanalog, as well as the numerical, recording.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the performances ofthe recorder.

The free length of the tape between the scanner and the capstan orcapstans is greatly reduced if the capstan or capstans is/are providedin close proximity to the scanner. In this case, it is necessary to takemeasures for facilitating the loading of the tape. This is obtained bymaking the capstan displaceable in rails, such that it is capable ofbeing shifted from one position where it engages the tape in thecartridge to a working position near the scanner. A variant of theinvention aims to realize recordings at variable speed. In this case, itis necessary to correct the angle error of the tracks recorded on thetape, due to the modification of the longitudinal speed of the tape.This correction is realized in the present invention by the adjustmentof the angle of the helix of a helical ramp or guide of the tape, havingvariable geometry and located in the stationary part of the scannerand/or by the axial displacement of a recording head on the scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the working position of the capstans at close proximity tothe scanner,

FIG. 2 shows the capstans engaged in the magnetic tape cartridge,

FIG. 3 shows an intermediate position of the capstans,

FIG. 4 shows the winch utilized for the displacing the capstans towardthe scanner,

FIG. 5 shows the guiding rails of the capstans,

FIG. 6 shows cross-sectional side views of the capstan and its motor,

FIGS. 6A and 6B show breakaway views of portions of FIG. 6,

FIG. 7 shows a passive capstan,

FIGS. 8A and 8B show the scanner with the helical ramp,

FIG. 9 shows the face of a tape on which recordings have been made byrotating heads,

FIG. 10 shows the recorded tracks in the case of a moving speed of thetape reduced by 50%,

FIG. 11 shows the recorded tracks in the case of a moving speed of 1/4of the nominal speed,

FIG. 12 shows the axial displacement of the recording heads on thescanner for small moving speeds of the tape, and

FIGS. 13A and 13B show the construction of the guides 2 and 3 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the recorder with the capstans in working position. Therecorder comprises a first input capstan-guide 10 and a second outputcapstan-guide 12. The tape 30 leaves the cartridge 15 by the cartridgedelivery spool 1 and it is received in the cartridge receiving spool 14.At the outside of the cartridge 15, the tape passes on guides 2 and 3which comprise also the magnetic heads for the auxiliary longitudinaltracks which have to operate when the tape is not engaged on thescanner, for example during fast windings. The tape then passes ontensiometers 4 and 5 which are pulleys mounted on pivoting arms fortensioning the tape and measuring its tensile stress in order to informthe control circuits about the tensile stress in the tape. The tape isthen displaced in front of the principal magnetic heads 6 and 7 forerasing, recording or reproducing the information recorded in thelongitudinal tracks. Elements 8 and 9 represent non rotatable guides ofcomplex form permitting a change of plane. The tape in the cartridge andon the elements 2 to 7 moves in parallel with the base of the device. Onthe other hand, from the guides 8 and 9 it comes down on one side andgoes up at the other side in order to join the helix of the scanner. Theguides 8 and 9 provide for constant tensile stress toward the top aswell as toward the bottom of the tape. Each of these guides may consistof two warped cylinders. Between the two capstans 10 and 12, the tapepasses on a rotatable drum 11 of the scanner which may comprise aplurality of magnetic heads. 13 illustrates a coding motor of the outputcapstan 12.

FIGS. 13A and 13B show the construction of the tape-guides 2 and 3. Inany driving device of the tape, the guiding of the latter is a difficultproblem to solve. Indeed, it is necessary to avoid floating of the tapecaused by a lateral play. It is also necessary to avoid a frictionbetween guide and tape which would produce an undesirable loss oftensile stress as well as longitudinal, particularly disturbingvibrations. This is the reason why one does not use stationary guides.In the present invention, the guides are comprised of two parts, arotatable slightly conical part 111 with an angle of conicity of e.g. 45minutes and a fixed part 112 on which abuts the tape 30 and whichpossibly bears an auxiliary recording or reproducing head 113. Thepresence of the cone has the effect of drawing the tape toward thebottom, against the fixed part so that any lateral play is avoided.Moreover, the friction is also diminished because the tape moves on theupper, rotatable part 111.

FIG. 2 shows the loading mechanism of the tape. The cartridge 15 is atfirst inserted horizontally in the apparatus and then it is lowered.During the phase of lowering, its door (not represented) opens and it isengaged by the capstans 16 and 17 and by the rolls of the tensiometers22, which pass behind the tape 30. The rolls of the tensiometers 22driven by a cable coupled to the same winch as the capstans move thenalong the paths 20 and 21 in order to reach their working position. Inthe present invention, the tensiometric arms have also the function ofextracting the tape from the cartridge. During this operation, thecapstans 16 and 17 come also out of the cartridge 15 and they areshifted toward the scanner 50 in accordance with the paths indicatedschematically by the arrows 18 and 19, but the transmission is sodesigned that they stop before coming into contact with the tape 30, asshown in FIG. 3. This position of the tape is utilized to rewind thetape at great speed without the latter rubbing on the scanner 50 of FIG.3. The rotatable guides 2 and 3 comprise also auxiliary magnetic headsfor the track bearing the time code track which permits a reading duringthe windings.

FIG. 4 shows the mechanism for the shifting of the capstans toward thescanner 50. The capstans are shifted by a cable 27 driven by a winch 28.In a tape driver with helical scanner, almost all the axes are warped.Now, it is important for avoiding damaging of the tape, to deal with itlike a flat surface, to allow for a small torsion, above all uniformlydistributed on the width of the tape (not surpassing generally an angleof one degree for a length equal to the width of the tape. In order toobtain this result, the axis or axes of the capstan or capstans musttake well determined inclinations at each position during theirdisplacement. In the present invention, the supports of the bearings ofthe capstans which are in practice their motors, are made each slidingor rolling on three independent rails 31-33 and 34-36 as illustrated inFIG. 5, the variable heights along these rails being so designed thatthe axes of rotation of the capstans take the required spatialorientations. Each capstan motor comprises three feet terminated byspheres at its lower part. These spheres slide in the rails 31 to 36which are in form of tubes with a longitudinal slit. This arrangementpermits the rails to have very complicated configurations and moreparticularly to guide them up or down. This provides not only therequired changes of levels to permit the joining of the horizontal pathof the tape in the cartridge to the helical path of this same tape onthe scanner, but also provides at each position the optimal inclinationof the capstan in order to avoid damaging the tape.

When the capstan arrives at its working position in close proximity tothe scanner, it is locked in this position by V-shaped upper abutments24 and 26 illustrated in FIGS. 1 and 2. This is feasible because therails are splayed at the location which corresponds to this workingposition, which permits a certain play to the capstan. The axis of thecapstan is terminated at both extremities by truncated spheres, which inthe normal working position of the capstans, come in contact with theupper abutments 24 and 26 thus positioning these capstans with greataccuracy while tolerating changes of the orientation of their axis. TheV-shaped upper abutments 24 and 26 are shown in FIG. 1. Mobile clasps 23and 25 illustrated in locking position in FIG. 1 and in open position inFIG. 2, press the extremities of the axis of the capstan against theseabutments. The lower abutments 37 and 38 are illustrated in FIG. 5. Theelements 39 and 40 are mobile cams the rotation of which displaces thelower extremity of the axis tangentially to the scanner and consequentlyalso its angle of orientation with respect to the axis of the scanner.This adjustment which can be servo-controlled permits correcting oferrors of geometry due to the modification (which will be describedlater on) of the angle of the helix of a guiding ramp for the tapehaving variable geometry as well as optimizing the movement of the tapeas a function of its orientation, speed, characteristics, etc. Theadjustment can be controlled by transducers which measure the pressurewith which, in the example described, the lower edge of the tape presseson the helical ramp or also the space between this edge of the tape andpoints of reference. Indeed, the modification of the angle of the axisof the capstan causes the tape to go up or down on the scanner. Theelements 41 and 42 of FIG. 5 are springs for taking up the play whichposition the extremity of the axis of the scanner in the radialdirection with respect to the scanner.

FIG. 6, 6A and 6B shows a sectional view of the capstan 10 and of itsmotor with the rotor 76. One recognizes the abutment 24 with itsfastening element 23 as well as the abutment 37 and the rotatable stopcam 39 controlled by a lever 84 and which permits tilting of the fixedaxis 72 of the capstan by a servo-mechanism not represented in order toimprove the movement of the tape. The axis 72 of the capstan terminateson each side by truncated spheres 78 and 79 which are provided in theabutments 24 and 37. FIG. 6 shows also spheres 85 and 86 terminating thefeet (3 feet) of the motor, these spheres sliding in the rails 87 and 88of tubular form with a longitudinal slit as mentioned previously, theserails guiding the motor of the capstan during its shifting from theposition illustrated in FIG. 2 toward the working position. A rotatabletransformer 77 transmitting to the stator the signals delivered by theencoder measuring the speed and the position of the rotor. 89 representsthe frame of the device. The capstan 10 rotates on a ball bearing 74which is prestressed by a permanent magnet 75. Indeed, in order to workcorrectly, a cylindrical ball bearing must support a certain prestressfor ensuring that the balls rotate instead of sliding. Usually, thisprestress is obtained using a second ball bearing and springs.

In our case, it is very important that the friction which opposes therotation be as small and constant as possible because the torque of themotor is determined by the measurement of the current consumption. Thevalue of this torque is utilized for controlling the tensile stress ofthe tape at the location of the spools. Indeed, the capstan can bewithout pressure roll. Then, the difference of tensile stress of thetape before and after the capstan must be small in order to avoidsliding. To this end, the motor, which supports the spool, pulls more orless strongly on the tape and it is the torque of the capstan whichcontrols this tensile stress. When replacing the second ball bearing bya magnetic prestress which is practically without losses, one reducessubstantially the losses by friction.

The recorder according to the invention accepts also passive capstans.Usually, the capstans are active which means that being driven by amotor, they contribute to the drive of the tape. But it is also possibleto utilize passive capstans which are driven by the tape and have thefunction to stabilize the speed of the tape. This can be done by puremechanical means combining e.g. the inertia and attenuating devices orby electronic means such as a combination of speed sensors or of sensorssensing variations of the speed and motors which behave like artificialmasses with attenuations. FIG. 7 shows schematically a passive capstan10 coupled to an inertial mass 45 in which is provided a rotatable mass46 bathed in oil 47 and serving as an attenuator.

The preceding shows that the loading of the tape is greatly facilitatedby the shifting of the capstans which is foreseen between the positionat the inside of the cartridge in which the tape passes without anydifficulty on the capstans 16 and 17 and the tensiometers 4 and 5,between the delivering and receiving coils 1 and 14. When the tape is sodisposed, the shifting of the capstans from the positions 16 and 17 tothe respective positions 10 and 12 permits an automatic setting in placeof the tape in the recorder.

In the recorders with a helical scanner, there is also a to solve theproblem of guarantying that at the time of a subsequent reading, thereading head passes exactly on the tracks recorded previously. To thisend, it is advantageous to guide laterally the tape during its travel onthe scanner. A universally utilized method consists of providing the nonrotatable drum of the scanner with a helical guide called a "ramp" onwhich the tape leans. The direction of the recorded tracks on the tapewith respect to its lower edge which is generally taken as a reference,is determined by the vectorial combination of the scanning movement ofthe rotatable head or heads and of the movement of translation of thetape itself. In order to read a tape moving at a speed different fromthe one utilized for the recording, and more particularly a tape at astandstill (viewing of a stationary image), the reading head has tofollow a trajectory somewhat different from the one that it followsnormally, in order to compensate the partial or total absence ofmovement or translation of the tape. A well known method consists ofoperating with a reading head ,not rigidly fixed to the rotatable drumof the scanner, in order to correct the error mentioned above withrespect to the trajectory. These movements of the heads are very rapidbecause they are bound to take place at the rhythm of the rotation ofthe scanner with a very sudden return for the beginning of the followingtrack. However, these movements (e.g., system AST Ampex) are not to beconfounded with the controlled, slow displacements which are very muchless capable of being disturbed by the accelerations and which will bediscussed later on. This known method is well suited for the apparatuseswith few heads which operate in an environment not submitted toimportant accelerations like the ones which arise in the aircrafts.

If the problem is not to read but to record at variable speed, theabsolute positioning of the recording head causes very serious problemsbecause of the few place and energy at disposal in the scanner andfurther because of the radial (up to 6000 g) and axial accelerations.Another known solution of this problem consists of inclining the axis ofrotation of the rotatable drum of the scanner with respect to the fixeddrum comprising the ramp. This solution is acceptable for small ormoderate wrap angles.

In the device according to the present invention, the non rotatable drumcomprising the helical guide or ramp is not a rigid element with a fixedhelix but a resiliently deformable piece in order to permit a relativelyweak modification of the angle of the helix. This deformation isnormally executed by means of cams or independent servo-motors. Thispermits one to effect the correction of the tracking recording as wellas while reading. It is easy to construct the device such that it willnot exaggeratedly be disturbed by the accelerations and thisconstruction is substantially more simple and more reliable than the onewith rapid, mobile heads, principally if the latter are numerous.

FIGS. 8A and 8B show a scanner with a helical ramp according to theinvention, the angle of the helix being capable of being modified byresilient deformation of the fixed drum of the scanner. The scannercomprises an upper, rotatable drum 11 on which the tape is helicallywound and a lower, non rotatable drum 51 bearing a helical ramp 53. Thedrum 51 can be resiliently deformed by the rotation of a cam 52 forpermitting a small modification of the step of the helix, typically of45 micrometer in the direction of the vertical arrows. This modificationof the step of the helix is equivalent to a modification of the angle ofthe helical ramp. FIG. 8A shows that the rotatable drum 11 bears atachometer wheel 62. This wheel is associated with a tachometric head64, shown in FIG. 8B for detecting and evaluating the speed of rotationof the drum. As shown in FIGS. 8A and 8B, transducers 61 and 63 deliverone pulse per turn of the drum as a synchronizing pulse. The rotatabledrum further comprises a magnetic recording or reading head 57 with itsceramic support 58. The head 57 can be adjusted vertically by a fineadjustment cam 59 and in the direction or azimuth by a fine adjustmentcam 60. Blade springs 56 force the ramp bearing drum 51 to rest on thebase 54 and resilient blades 55 provide for the centering and theangular positioning of the lower fixed ramp bearing drum 51. 65 is thedriving motor of the rotatable upper drum 11.

The modification of the angle of the ramp of the helix necessitates acorrection of the geometry of the movement of the tape. In the deviceaccording to the invention, this correction is executed by a smallmodification coordinated with the movement of the mobile ramp, of thedirection of the axis of the capstan-guides. This adjustment is executedby means of the cam 39 of FIG. 6 and of its control lever 84.

Let us now examine the conditions bound to the recordings at variablespeed without notable modification of the relative speed between headand tape, modification which causes a corresponding modificationdifficult to be realized by the equalization circuits. For suchrecordings, by utilization of the ramp of variable geometry according tothe invention or the known method of the axial displacement of the headsfor correcting the tracking error (AST), it is easy to record at a wholefraction of the nominal speed (1/2, 1/3, 1/4, etc.), the head executingone movement during which recording takes place and a certain number ofvoid movements, without recording.

For certain applications, these fractional steps of speed are toocoarse. It is possible to make them finer by switching the heads, whichmeans by altering their natural working order. It is then necessary todisplace them very slightly axially in order to correct the trackingerror.

FIG. 9 shows the recorded face of a tape by means of rotatable heads.The magnetic tape or the tape of a different kind 30 moves according tothe direction 92, from the right hand side to the left hand side in theFigure. It is scanned by heads A, B, C and D (four in this example)which, if the tape would be stationary, would follow the path 93, bydisplacing themselves in the direction 94 which makes an angle 95 withthe lower edge of the tape. But, as during the duration of one sweep byone by the heads, the tape has been shifted of the distance 96, thetrack recorded on the tape is represented by 97, this track making anangle 98 with the edge of reference of the tape.

These tracks 97 on the tape which are normalized and at the time ofrecording at different speed of the tape, the track recorded on thelatter must, always be identical to the one which would be recorded inthe nominal conditions.

Let us suppose now that the tape moves at a speed lower than the nominalspeed. During a sweep, the advance 96 will be smaller. In order for thetrack 97 on the tape to be of the normalized angle, it is necessary thatthe path 93 from the head has an angle which differs of the nominalangle. This can be done by an axial motion of the head (method AST) orby a modification of the guiding angle by the ramp of variable geometryin accordance with the invention.

For the further description, one can admit that for each moving speed ofthe tape one adopts a determined angle of the ramp in order for thetrack on this tape to be of the correct angle. It is then sufficient forthe following description to consider only the point 99 of the beginningof the track.

In the example of FIG. 9, four recording heads A, B, C and D actsuccessively on the tape. For one turn of the scanner, four tracks, oneper head are recorded.

If it is assumed that the heads are always fed in this same order (A, B,C and D) for recording their bursts, the next step of speed that it ispossible to obtain is 1/2. The tape must then move at half the nominalspeed, the angle of the ramp or the angle obtained articifially by amovement of the heads would be according to the track 100 of FIG. 10.Its track, on a stationary tape would be 93', different from the track93 of FIG. 9. But, as the longitudinal advance is halved like 96', thetrack 100 on the tape is again conformed to the normalized track 97.

At nominal speed, the next track would be recorded by the head B whichstarts its path at point 99 of FIG. 9. However, on FIG. 10, the tape hasnot advanced by the value 96 of FIG. 9 but in our case of the halveredvalue 96' of FIG. 10. If the head B would have been active, it wouldhave started its path at point 99 of FIG. 10, which is not permittedbecause this path would run over the path of the head A. Hence, in thiscase, the head B will be rendered inactive and it would effect a voidpassage without recording. The next active head will be the head C whichwill record a track in parallel to the one of the head A track which, atnominal speed, would have been recorded by the head B. This exampleshows the possibility to record at half the nominal speed by renderinginactive one in two heads. Similarly, it would be possible to operate at1/4 of the nominal speed by rendering inactive three in four heads. FIG.11 represents this case and it is visible that only the head A wasactive. If the recording speed should be 1/8 of the nominal value, asingle head would be recording every two sweeps. It is therefore easy toobtain recording speeds stepped in octaves.

However, if one seeks to obtain smaller steps of speed it is necessaryto switch on the heads in a different way. Let us examine FIG. 12. It isdesired to obtain tracks 101, 102, 103 and 104 of FIG. 12 with a movingspeed comprised between 1/2 and 1/1 of the nominal value or 3/4 of thenominal value in this example. Head A records the trak 101 in a usualmanner. When head B should start the recording on the tape, the latterhas advanced of only 3/4 of the nominal speed so that the head is not inthe good position for recording the track 102. The Figure shows that onemust displace axially the head B on the scanner of the amount indicatedin FIG. 12.

The axial displacement of one head could give to think that this methodis the same as the method AST. There exists however a fundamentaldifference between these two methods. The method AST has to be veryrapid because the displacement is executed at each turn of the scanner,that is in a few milliseconds while in the method according to theinvention, this displacement is done one for all at the time of choosingthe speed before the recording. It can be done by a cm system (seemagnetic head 57 with fine adjustment cam for the height of the head 59in FIG. 8) drived by a servo-motor hence capable to support theaccelerations and vibrations to which is submitted an embarked recordingapparatus.

Hence, the head B has been displaced by the value 105 in FIG. 12 whichpermits this head to record the track 102. In its turn, the head C isdisplaced by the value 106 and it records the track 103. The track 104could have been recorded by the head D displaced by the value 107.However, this is useless because this track 104 will be recorded by thehead A after one turn of the scanner because then this head A comes tocoincide exactly with the track 104 when the tape has advanced of 4segments of 3/4 of nominal length each. Head D will effect a voidpassage, being not activated.

The preceding shows that the possibility according to the invention todisplace axially the recording heads according to the desiredlongitudinal speed of the tape permits one to realize recordings withsteps smaller than once octave. The axial displacement being adjustedone for all times before the recording as a function of the moving speedof the tape. On the other hand, the angle of the track on the tape isdetermined by the angle of the ramp as a function of the moving speed ofthe tape. Both measures lead to a construction of the device capable toresist advantageously to relatively high accelerations and vibrations.

We claim:
 1. A device for recording/reproducing signals comprising atleast one rotatable head for recording/reproducing said signals byhelical scanning, comprising:a flexible support having a plurality oftracks on which said signals are recorded/reproduced by said rotatablehead; a scanner with a rotatable drum bearing said at least one head,said flexible support being wound and shifted helically around saidrotatable drum, said scanner further comprising a stationary drumbearing a helical ramp for laterally guiding said flexible supportduring its helical shifting around said rotatable drum; and said devicefurther comprising means for producing a resilient deformation of saidstationary drum to enable modification of the step and angle of thehelix of the helical ramp to correct for errors in the scanning of saidtracks due to a modification of the moving speed of said flexiblesupport.
 2. A device according to claim 1, further comprising capstansand means for adjusting the inclination of the axis of said capstansrelative to the axis of said scanner to thereby optimally adjust thegeometry of the motion of said flexible support.
 3. Arecording/reproducing device according to claim 1, wherein said at leastone rotatable head comprisesa plurality of rotatable heads capable ofrecording at a plurality of speeds on said flexible support; said devicefurther comprising means for selectively activating said heads in apreferred order; and means for axially shifting said heads beforerecording on said flexible support, wherein the amount of axial shiftcorresponds to the recording speed to enable the path of said heads tocoincide with one of said tracks on said flexible support to enablerecording at speeds having a relative mutual separation smaller thanoctaves.